I know I'm late to respond to this, but it looks nice. I think Mario and Luigi need some shading to help them match the original art a bit more, though. And of course, gotta have that "Only For" message in the corner, since this is not a backwards-compatible game.
And nobody answered this: in regards to why some GBC games were backwards-compatible and others weren't, it came down to memory and complexity of the graphics. In the case of Pokemon Crystal, they added animations for all 250+ pokemon as well as other features, which left them no room for black & white version of the graphics (or for that matter, Super Game Boy graphics, which Gold & Silver had features for). And then there are games like Dragon Warrior III, which also had animated monster sprites as well as SNES-quality maps, making it doubly impossible for them to make the game work in four shades of grey.
The alternate patch won't have any unnecessary modifications, so there is no Luigi, the statusbar remains white, Fire Mario have a feather, the 1-UP is a heart, etc.
Wait, Mario won't have his feather in the main patch? Why not? I feel like it was about as iconic as the bunny ears.